This invention relates to the removal of noxious substances from gas streams, in particular the removal of very stable fluorocarbons from gas streams emanating from i-conductor processing chambers by means of combustion.
Many substances used in semi-conductor device manufacturing, and which are extracted from a chamber in which such manufacturing takes place, are toxic and/or environmentally harmful and must therefore be scrubbed from the exhaust gas stream before its release in to the atmosphere.
A number of different types of wet or dry chemical scrubbing reactors have been proposed and numerous are commercially employed in the semi-conductor industry.
For example, in our Patent Specification No. WO 89/11905 there is disclosed a dry chemical reactor sold by our Edwards High Vacuum International Division comprising a heated packed tube of granular substances through which the exhaust stream is directed including in particular a first stage of silicon (with an optical addition of copper when the exhaust stream contains nitrogen trifluoride in particular) and a second stage of calcium oxide commonly in the form of lime. Such a reactor has met with considerable commercial success for the scrubbing of such toxic substances.
It is also known from European Patent Specification No 694 735 in the name of Alzeta Corporation, the contents of which are incorporated herein by reference, that noxious substances of the type in question can be removed from exhaust streams by combustion.
There is described in this prior Specification a process for the combustive destruction of noxious substances which comprises injecting an exhaust gas and added fuel gas in to a combustion zone that is laterally surrounded by the exit surface of a foraminous gas burner, simultaneously supplying fuel gas and air to the burner to effect combustion at the exit surface, the amount of the fuel gas supplied to the foraminous gas burner being on a BTU basis, greater than that of the added fuel gas, and the amount of the air being in excess of the stoichiometric requirement of all the combustibles entering the combustion zone, and discharging the remitting combustion product stream from the combustion zone.
A central feature of the prior combustive process is the critical need to supply the fuel gas admixed with the exhaust gas stream in to the combustion zone of the burner. Such premixing of the fuel gas and exhaust gas stream allows for a much greater and efficient scrubbing of the perfluorocarbon hexafluoroethane (C.sub.2 F.sub.6). However, there remains certain problems associated with the scrubbing of the even more stable perfluorocarbon tetrafluoromethane (CF.sub.4).
A great advantage of the prior combustive scrubbing process described above is that it inherently limits the maximum temperature that can be attained in the combustion chamber and thereby suppress the formation of NO.sub.x gas by-products that may otherwise be formed.
However, the relatively low maximum temperature may become a limiting factor in the destruction of the most stable perfluorocarbon gases, in particular tetrafluoromethane (CF.sub.4).